Polymers and copolymers of vinylidene chloride with such comonomers as acrylonitrile, vinyl chloride and lower alkyl acrylates have found wide use in the manufacture of filaments, sheets, tubes, films and extruded and molded shapes. It is known that such polymers are difficult to work with due to their brittleness, relatively poor flowing qualities and susceptibility to thermal degradation, as evidenced by the development of discoloration and the presence of gas bubbles during the fabrication thereof, and particularly during the thermal extrusion of film materials therefrom. It has, heretofore, been common practice to incorporate conventional plasticizers into vinylidene chloride polymer compositions to improve their workability. Thus, citric acid esters (see British Pat. No. 739,411), sebacic acid esters (see U.S. Pat. No. 2,604,458) and/or phthalate esters (see British Pat. No. 811,532) have been utilized in combination with vinylidene chloride polymers to produce film materials. Polymer compositions containing such plasticizers in conjunction with various stabilizer systems, e.g., combinations of an epoxidized soybean oil and an oxide of the metals of Group II of the periodic table, such as magnesium oxide, have also been used as disclosed, e.g., in U.S. Pat. No. 3,261,793.
It is known that these plasticized vinylidene chloride polymer compositions are less than wholly satisfactory for a number of reasons, e.g., most of the known plasticizers have poor compatibility with vinylidene chloride polymer compositions and migrate to the surface of articles, such as films, produced therefrom. This result is especially disadvantageous wherein the polymeric film material is used in food wrapping applications. Further, the presence of such plasticizers often significantly reduces the tensile strength and tends to increase the gas permeability of polymeric films produced from the vinylidene chloride polymers and particularly the permeability to water vapor and air. Further, films produced from the normally crystalline vinylidene chloride polymers containing the conventional plasticizers are often characterized by the presence of gas bubbles which are detrimental where clear, continuous film materials are required.
It is, therefore, the primary object of the present invention to produce vinylidene chloride polymer film materials characterized by impermeability to gases such as water vapor and air and which, in addition, can be thermally fabricated, e.g., extruded into film form, in the substantial absence of discoloration and the formation of bubbles.